Move load_timeline_metadata into separate function (#9080)

Moves the per-timeline code to load timeline metadata into a new
dedicated function called `load_timeline_metadata`. The old
`load_timeline_metadata` becomes `load_timelines_metadata`.

Split out of #8907

Part of #8088

pageserver/src/tenant.rs

@@ -18,7 +18,6 @@ use camino::Utf8Path;
 use camino::Utf8PathBuf;
 use enumset::EnumSet;
 use futures::stream::FuturesUnordered;
-use futures::FutureExt;
 use futures::StreamExt;
 use pageserver_api::models;
 use pageserver_api::models::AuxFilePolicy;
@@ -34,6 +33,7 @@ use remote_storage::GenericRemoteStorage;
 use remote_storage::TimeoutOrCancel;
 use std::collections::BTreeMap;
 use std::fmt;
+use std::future::Future;
 use std::sync::Weak;
 use std::time::SystemTime;
 use storage_broker::BrokerClientChannel;
@@ -1031,13 +1031,9 @@ impl Tenant {
         }
 
         Ok(TenantPreload {
-            timelines: Self::load_timeline_metadata(
-                self,
-                remote_timeline_ids,
-                remote_storage,
-                cancel,
-            )
-            .await?,
+            timelines: self
+                .load_timelines_metadata(remote_timeline_ids, remote_storage, cancel)
+                .await?,
         })
     }
 
@@ -1303,41 +1299,18 @@ impl Tenant {
         .await
     }
 
-    async fn load_timeline_metadata(
+    async fn load_timelines_metadata(
         self: &Arc<Tenant>,
         timeline_ids: HashSet<TimelineId>,
         remote_storage: &GenericRemoteStorage,
         cancel: CancellationToken,
     ) -> anyhow::Result<HashMap<TimelineId, TimelinePreload>> {
         let mut part_downloads = JoinSet::new();
         for timeline_id in timeline_ids {
-            let client = RemoteTimelineClient::new(
-                remote_storage.clone(),
-                self.deletion_queue_client.clone(),
-                self.conf,
-                self.tenant_shard_id,
-                timeline_id,
-                self.generation,
-            );
             let cancel_clone = cancel.clone();
             part_downloads.spawn(
-                async move {
-                    debug!("starting index part download");
-
-                    let index_part = client.download_index_file(&cancel_clone).await;
-
-                    debug!("finished index part download");
-
-                    Result::<_, anyhow::Error>::Ok(TimelinePreload {
-                        client,
-                        timeline_id,
-                        index_part,
-                    })
-                }
-                .map(move |res| {
-                    res.with_context(|| format!("download index part for timeline {timeline_id}"))
-                })
-                .instrument(info_span!("download_index_part", %timeline_id)),
+                self.load_timeline_metadata(timeline_id, remote_storage.clone(), cancel_clone)
+                    .instrument(info_span!("download_index_part", %timeline_id)),
             );
         }
 
@@ -1348,8 +1321,7 @@ impl Tenant {
                 next = part_downloads.join_next() => {
                     match next {
                         Some(result) => {
-                            let preload_result = result.context("join preload task")?;
-                            let preload = preload_result?;
+                            let preload = result.context("join preload task")?;
                             timeline_preloads.insert(preload.timeline_id, preload);
                         },
                         None => {
@@ -1366,6 +1338,36 @@ impl Tenant {
         Ok(timeline_preloads)
     }
 
+    fn load_timeline_metadata(
+        self: &Arc<Tenant>,
+        timeline_id: TimelineId,
+        remote_storage: GenericRemoteStorage,
+        cancel: CancellationToken,
+    ) -> impl Future<Output = TimelinePreload> {
+        let client = RemoteTimelineClient::new(
+            remote_storage.clone(),
+            self.deletion_queue_client.clone(),
+            self.conf,
+            self.tenant_shard_id,
+            timeline_id,
+            self.generation,
+        );
+        async move {
+            debug_assert_current_span_has_tenant_and_timeline_id();
+            debug!("starting index part download");
+
+            let index_part = client.download_index_file(&cancel).await;
+
+            debug!("finished index part download");
+
+            TimelinePreload {
+                client,
+                timeline_id,
+                index_part,
+            }
+        }
+    }
+
     pub(crate) async fn apply_timeline_archival_config(
         &self,
         timeline_id: TimelineId,